The present invention relates to an image sensor chip for use as a component of an image reading apparatus. It also relates to an image reading apparatus employing such an image sensor chip.
An image sensor chip is known which is elongated in a direction and which has a surface portion integrally incorporating a plurality of light receiving elements for photoelectric conversion. Each of the light receiving elements has a light receiving surface for receiving light and for generating an image signal with an output level corresponding to the received amount of light. The respective light receiving surfaces serve as the picture elements of the image sensor chip and are arranged in a row at a constant pitch longitudinally of the image sensor chip.
The length of such an image sensor chip ranges from several millimeters to less than 20 millimeters, so that a single image sensor chip fails to read a document which is wider than it. Therefore, in actual use of the image sensor chip as a component of an image reading apparatus, a plurality of such image sensor chips are arranged in an array.
When using a plurality of image sensor chips in this way, they may suffer fluctuations in the output level of their image signals due to manufacturing errors of the image sensor chips and positional deviations relative to the lens of the image reading apparatus even if the light receiving elements receive an equal amount of light. Accordingly, image reading is performed by a plurality of image sensor chips, the image signals outputted from the plurality of light receiving elements are subjected to level correction to prevent unevenness in the output level. In this regard, if the level correction is performed individually for all signals from the light receiving elements, the process will be very cumbersome. Therefore, as a simple way to perform the level correction, the correction is performed collectively for each group of e.g. eight light receiving elements, wherein the image signal outputted from one of the eight light receiving elements in the group is used as the basis for the correction.
On the other hand, the number of the light receiving elements incorporated in a prior art image sensor chip is, for example, 32, 64 or 128 which is an integer multiple of 16. This is convenient for the above-described level correction because the light receiving elements of the image sensor chip can be exactly divided into plural groups of eight elements without any excess.
However, the prior art image sensor chip still has the following problems.
For image reading of a document by using an array of image sensor chips, the image sensor chip array is required to provide an image reading width (substantially equal to the combined length of the image sensor chips in the array) which is greater by an suitable excess than the document width. If the image reading width is excessively greater than the document width, the image sensor chips may be partially wasted, and the image reading apparatus becomes unnecessarily large. Conversely, if the excess of the image reading width is too small, the image sensor chip array may fails to read a side part of the document even due to a slight deviation of the document positioning. Here, in order for the above-described excess to be suitable, it should be relatively smaller for a narrower document but relatively larger for a wider document. This is because, in reading a narrow document accurately, the document is unlikely to positionally deviate too much in the primary scanning direction (or in the direction of the row of the image reading picture elements), whereas, in reading a wide document, the document is likely to positionally deviate in the primary scanning direction to a large extent.
However, the prior art image sensor chip is incapable of meeting the requirements. Taking an image sensor chip of 10.84 mm in length with a resolution of 11.8 dots/mm as an example, it has 128 image reading picture elements. In order for such an image sensor chip to meet a variety of reading width requirements, the specifications shown in Table 1 below are called for.
Taking an image sensor chip of 8 mm in length with a resolution of 8 dots/mm as another example, it has 64 image reading picture elements. In order for such an image sensor chip to meet various reading width requirements, the specifications shown in Table 2 below are called for.
As appreciated from Tables 1 and 2, when using a given number of conventional image sensor chips in an array for meeting the required reading width S, the excess Lb of the chip array length which is the difference between the required reading width S and the chip array length La becomes smaller as the required reading width S increases. Similarly, the excess ratio (R=Lb/S) also becomes gradually smaller. In this way, the use of the conventional image sensor chips directly contradicts the above-mentioned requirements, making it difficult to suitably adjust the excess Lb to the required reading width S. As a result, if an attempt is made to use a minimum number of conventional image sensor chips for saving, even a slight deviation of the document may cause difficulty in performing appropriate image reading due to an unduly small excess Lb. Further, the use of an excessive number of image sensor chips for overcoming such a problem will result in added cost while causing a size increase of the image reading apparatus as a whole.
It is therefore an object of the present invention to provide an image sensor chip which is capable of eliminating or alleviating the problems described above.
It is another object of the present invention to provide an image reading apparatus employing such a image sensor chip.
According to a first aspect of the present invention, there is provided an image sensor chip comprising: a plurality of light receiving elements for photoelectric conversion, the plurality of light receiving elements having light receiving surfaces serving as a plurality of image reading picture elements, the plurality of image reading picture elements being arranged in a row at a constant pitch on the chip; wherein the number N of the image reading picture elements is defined as N=nxc2x7k (other than an integer multiple of 16); wherein n represents the number of image signals processed as a group when the image signals from the light receiving elements are subjected to level correction on a group-by-group basis; wherein k is a minimum integer satisfying 0 less than (nxc2x7kxe2x88x92xcex3xc2x7p) less than n; wherein xcex3 is a resolution defined as the number of image reading picture elements per predetermined unit length; and wherein p is a minimum difference between required reading widths as calculated on the basis of said unit length.
Preferably, the chip comprises a semiconductor chip which as a rectangular shape elongated in a direction, and the plurality of light receiving elements are built in the chip as arranged in said direction.
Preferably, the image sensor chip further comprises a built-in circuit for operating the plurality of light receiving elements.
Preferably, n is 8, xcex3 is 11.8 dots/mm, and the number N of the reading picture elements is 152.
Preferably, n is 8, xcex3 is 8 dots/mm, and the number N of the image reading picture elements is 104.
According to a second aspect of the present invention, there is provided an image reading apparatus comprising: a plurality of image sensor chips carrying a plurality of light receiving elements for photoelectric conversion, the plurality of light receiving elements having light receiving surfaces serving as a plurality of image reading picture elements, the plurality of image reading picture elements being arranged in a row at a constant pitch on the chip; a substrate carrying thereon the plurality of image sensor chips arranged in an array; a light source for irradiating a reading line area of a document image with light; a lens for focusing the light, which is irradiated by the light source and reflected from the document, at the plurality of image reading picture elements; wherein the number N of the image reading picture elements is defined as N=nxc2x7k (other than an integer multiple of 16); wherein n represents the number of image signals processed as a group when the image signals from the light receiving elements are subjected to level correction on a group-by-group basis; wherein k is a minimum integer satisfying 0 less than (nxc2x7kxe2x88x92xcex3xc2x7p) less than n; wherein xcex3 is a resolution defined as the number of image reading picture elements per predetermined unit length; wherein p is a minimum difference between required reading widths as calculated on the basis of said unit length.
As the number N of the image reading picture elements satisfies the condition described above, the present invention has following advantages.
Firstly, when arranging a plurality of image sensor chips in an array for providing any one of the required reading widths which is an integer multiple of p, it is only necessary to use only a minimum number of image sensor chips whichever reading width is selected, whereby the excess of the image sensor chip array relative to the actual reading width can be kept at a constant ratio relative to the selected reading width. Therefore, the present invention allows the excess of the image sensor chip array to be smaller for a narrower document but larger for a wider document. Consequently, according to the present invention as opposed to the prior art, when using a plurality of image sensor chips in an array for providing a desired reading width, it is possible to reduce a waste with respect to the number and length of the image sensor chips, thereby reducing the cost while realizing a size reduction of the image reading apparatus incorporating such image sensor chips. Further, since the reading width of the image sensor chip array still has a reasonable excess, it is possible to avoid an image reading difficulty which may be caused due to a positional deviation of the document deviation.
Secondly, the number N of the image reading picture elements is an integer multiple of the number of the image signals processed together in a single group for group-by-group level correction, the plurality of image signals outputted from each image sensor chip can be divided into a plurality of groups without any remainder for the level correction. Therefore, according to the present invention, the plurality of image signals outputted separately from two image sensor chips need not be grouped together for correction even though the plurality of image sensor chips are arranged in an array. Thus, it is possible to avoid the disadvantage previously encountered with the prior in level correction.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.